1. Field of the Invention
The invention relates to a process for producing polycrystalline silicon.
2. Description of the Related Art
Polycrystalline silicon (polysilicon) is predominantly deposited on thin rods from halosilanes such as trichlorosilane by means of the Siemens process, which gives polycrystalline silicon rods, which are then comminuted into polycrystalline silicon chunks. After the comminution into chunks, it is customary to classify these into particular size classes. After sorting and classification, the chunks are dosed to a particular weight and packed in a plastic bag. A corresponding method for sorting, classification, dosage and packing of chunks is known from US 2013309524 A1.
For applications in the semiconductor and solar industries, chunk polysilicon with a minimum level of contamination is desired. It is therefore desirable that the comminution into chunks, sorting and classification, dosage and packing be effected with a minimum level of contamination.
Typically, the chunks have to be transported from one apparatus to another between the individual processing steps, for example from the comminution apparatus to the packing machine. In this case, it is customary to store the chunks intermediately in what are called buffer vessels, typically plastic boxes.
The last processing step is always packing in plastic bags, but is problematic because of the fact that chunk polysilicon is a sharp-edged, non-free-flowing bulk material which can penetrate the plastic bag in the course of filling or in the worst case even completely destroy it.
DE 10 2007 027 110 A1 discloses, for this purpose, a method for packaging polycrystalline silicon, comprising filling polycrystalline silicon by a filling device into a freely suspended, completely formed bag, and closing the bag thus filled, wherein the bag consists of high-purity plastic with a wall thickness of from 10 to 1000 μm, where the filling device comprises a freely suspended energy absorber of a nonmetallic low-contamination material, which is introduced into the plastic bag before filling with the polycrystalline silicon and by means of which the polycrystalline silicon is filled into the plastic bag, and the freely suspended energy absorber is then removed from the plastic bag filled with polycrystalline silicon, and the plastic bag is closed.
Such a method, which provides for an energy absorber within the plastic bag, can largely prevent puncturing of the plastic bag during the packing. However, this applies only to small or light chunks.
It has been found that the risk of bag damage increases proportionally with the chunk mass.
One conceivable way in principle of reducing the puncture rate by strengthening the bag film has not been found to be very practicable, especially since such a less pliable film would be difficult to handle. The packing machines being used are not designed for films having a thickness greater than 350 μm. Moreover, it would take longer to weld such thick bags, which reduces the throughput.
Such puncturing of the bag can occur not just during the packing operation but also in the course of transport to the customer. Chunk polysilicon is sharp-edged, and so, in the event of unfavorable orientation of the chunks in the bag, relative movement of the chunks to the bag film and pressure of the chunks on the bag film result, respectively, in the chunks cutting through and penetrating the bag film.
Experience has shown that bags made from commercial PE films filled with chunk polysilicon exhibit weld seams that have been torn open in the course of or after transport.
Chunks protruding from the bag packing can be unacceptably contaminated directly by surrounding materials, and chunks inside as a result of inflow of ambient air. This problem is also manifested in the case of so-called double bags, in which the polysilicon is filled into a first bag and this first bag is subsequently introduced into a second bag.
For larger chunks, US 20130269295 A1 discloses a method for packing polycrystalline silicon in the form of chunks or round rods having a weight of greater than 2 kg and a size of 90 to 170 mm, wherein at least one film in each case is inserted into a cuboidal cardboard box matched to the dimensions of the polycrystalline silicon to be packed, the polycrystalline silicon is introduced into the at least one film, the at least one film subsequently being welded and enclosing the polycrystalline silicon. The polycrystalline silicon which is in the form of a chunk or round rod and has a weight of greater than 2 kg and a size of 90 to 170 mm is surrounded by at least one film which has a thickness of 10 to 200 μm and encloses the polycrystalline silicon, this at least one film being surrounded by a further film having a reinforcing structure. After the films have been welded, the polycrystalline silicon is introduced into a transport vessel comprising separating elements or cardboard boxes.
In this way, puncturing during transport is supposed to be avoided more effectively in the case of larger chunks.
In principle, the idea is thus to transport the chunks in plastic boxes and, as a final step, to pack them in plastic bags or films.